Silica fume exists in the form of ultra fine powder with a fineness having a specific surface of 5-100 m.sup.2 /g and a bulk density of 100-400 kg/m.sup.3. Namely, the silica fume is significantly fine as compared with Portland cement usually having a specific surface of 0.3 m.sup.2 /g. When the silica fume is added in concrete in combination with a superplaticizer and the so-called dispersant such as a high performance water reducing agent in kneading the concrete, the fine particles thereof are dispersed in the concrete and thus densely fill the voids of the concrete. Also, since the silica fume has a high activity and is in the form of ultrafine powder, it promotes the pozzolan reaction with cement in concrete. Further, even in the silica fume-rich blending, owing to the characteristic of the ultrafine powder, the silica fume contributes to easily manufacture the concrete having a small water-cement ratio and a high strength.
Neofume exists in the form of ultrafine powder, which is obtained by fly ash, with a fineness having a specific surface of 20-130 m.sup.2 /g, and has the same characteristics as the silica fume.
In ready mixed concrete factories, the ultrafine powder of this type has been steadily used in the form of: in U.S.A, being made in the slurry; or in Norway, for facilitating the handling thereof, being granulated by aeration with the air blown into a storage silo. In the case of being made in the slurry, since the blending ratio is determined by volume, the blending amount by weight of the added ultrafine powder is obscure, which is unsuitable for the actual standard in Japan. Also, in the case of being granulated, as a result of the research by the present inventors, it has been revealed that the silica fume cannot fully achieve the performance as the ultrafine powder in concrete (Effect of physical properties of silica fume on the properties of high strength concrete; Annual Report of Concrete Engineering, 13-1, pp. 291-296, (1991)).
When being stored in a silo, the ultrafine powder induces the bridging phenomenon, and is difficult to discharge therefrom. For preventing bridging in a vertical silo, there has been proposed the aerating technique described above: however, it is inconvenient as it granulates the ultrafine powder. Also, the ultrafine powder has a problem that, when being released in the air, it is suspended in the air and remarkably scattered. Further, in the case of discharging the ultrafine powder from the silo, transporting it, and mixing it in concrete, it is not easy to feed a specified amount of the ultrafine powder for a short time, automatically weighing it, and feeding it in a concrete mixer. The reason for this is as follows:
(1) The amount of the ultrafine powder discharged from the silo greatly deviates as a factor of time, according to bridging etc. PA0 (2) To vertically lift the ultrafine powder with a screw conveyor or the like, the conveyor must be usually filled therewith; however, the steady feed of the ultrafine powder is difficult to achieve. PA0 (3) Accordingly, it is necessary to provide an intermediate tank for storing a specified amount of the ultrafine powder therein and discharging therefrom. PA0 Also, in the case that the discharging line from the intermediate tank includes the vertically upward transportation of powder, the quantitative transportation must be contrived. PA0 (4) In the case of (3), even when the ultrafine powder is weighed before the transportation, the transported amount cannot be recognized, because it is affected by the amount remaining in the transporting conduit.
Also, generally, in the case of installing an ultrafine powder feeding apparatus on the existing concrete batcher plant, the installation is liable to be greatly restricted by the narrow installation space.
It is very difficult to handle the ultrafine powder. Conventionally, this has been solved by the slurry-making or granulating technique.